1. Field of the Invention
The present invention relates to an image processing system enabling clear reproduction of an original document including mixed characters, ruled lines, and photos. The characters, ruled lines, and photos in the original document are scanned by an image scanner and a scanned image is converted to a multilevel signal and further converted to a binary signal. The original document is clearly reproduced based on processing of the binary signal according to the present invention.
2. Description of the Related Art
Image processing systems are widely used in the field of information processing systems. In image processing, a character can be easily converted to a binary signal based on a predetermined threshold level because the contrast of light and shade thereof is very clear. That is, for example, when the character portion (black portion) is a value "1", the remaining portion (white portion) is a value "0". Accordingly, when all areas of the document comprise only characters, the image processing of the original document is very easy.
When characters, ruled lines, and photos are mixed on one sheet of the original document, the image processing thereof is very troublesome because the photos contain half-tone colors such as gray colors so that the multi-level signal includes gray color areas, and fine lines of characters or ruled lines also become gray.
One known method for processing half-tone colors is called "dithering". In the dither method, the light and shade level of each pixel of the original image are compared with a plurality of threshold levels (variable threshold level), and ON/OFF of the display elements are determined based on the resultant data after comparison. In this case, each of threshold levels is selected at random at every pixel by using, for example, a table of random numbers.
Japanese Unexamined Patent Publication (Kokai) No. 58-3374 discloses a method for processing a half-tone color based on the density level of the black pixel. That is, this method includes steps for dividing the original image into a plurality of blocks, and for obtaining the difference between a maximum density level and a minimum density level in each block. When the difference value is larger than a predetermined threshold level, the area of such a block is binary-coded by a constant threshold level as a binary image area. When the difference value is smaller than a predetermined threshold level, this area is binary-coded by dithering as a half-tone area.
Japanese Unexamined Patent Publication (Kokai) No. 60-196068 discloses a method for determining either the binary image area or the half-tone area. This method includes the steps for binary-coding of the scanned data based on a plurality of threshold levels, for counting the number of the values "0" and "1" in each threshold level, for comparing the counted numbers with each threshold level, and for detecting a change of the counted numbers by comparing them with a previous number. Whether the binary image area or the half-tone image area is to be selected is determined from this change of the counted numbers.